558 GENETICS IN RELATION TO AGRICULTURE 
female hybrids with the zebu, Bos indicus. With the yak, Bibos grun- 
iens; the gayal, Bibos frontalis, the gaur, Bibos gaurus, and the bison, 
Bison americanus, the female hybrids with the domestic cow are fertile, 
but the males are sterile. The banteng, Bibos sondaicus, and the zebu 
behave like this latter series in giving fertile female and sterile male 
offspring. In this respect they resemble Detlefsen’s and Castle and 
Wright’s results with species crosses among guinea-pigs, the female 
hybrids of which were fertile, the males sterile. 
Among domesticated birds in particular the reproductive powers are 
strongly disturbed by hybridization. Not only are such hybrids often 
Fig. 219.—Abnormal reduction divisions in spermatogenesis of the mule. (After 
W odsedalek.) 
sterile, but very frequently the sexual organs develop in an abnormal 
fashion strongly suggestive of intersexualism of the kind exhibited by 
Goldschmidt’s Lymantria hybrids. Smith and Thomas have examined 
sterile hybrids between species of pheasants. They found that very 
often ovarian degeneration or imperfect development occurs in the 
females, as a consequence of which a marked tendency exists to assume 
plumage patterns and characters peculiar to the male. 
Here we are dealing rather definitely with a type of sterility different 
from that which characterizes different families within a species or 
breed or different mutant types of Drosophila, the sterility here appears 
to be more deep-seated and strangely enough, far from being associated 
with a general diminution in vigor, the vigor and size of the hybrids are 
often very augmented. We are not surprised, therefore, to find that 
profound disturbances in the hereditary mechanism occur in such hybrids. 
Wodsedalek has shown that irregular reduction divisions occur in the 
mule (Fig. 219). Smith and Thomas have shown specifically that in 
sterile hybrid pheasants of both sexes the abnormal behavior and de- 
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